Catalyst System for a Multi-Walled Carbon Nanotube Production Process

ABSTRACT

The present invention relates to a catalyst system for the selective conversion of hydrocarbons into multi-walled carbon nanotubes and hydrogen comprising a compound of the formula: (Ni,Co)Fe y O z (Al 2 O 3 )w wherein ‘y’ represents the molar fraction of Fe relative to Co and Ni and wherein 0.11≦y≦9.0, 1.12≦z≦14.5, and 1.5≦w≦64

FIELD OF THE INVENTION

The present invention is related to a catalyst system for the conversionof hydrocarbons into carbon nanotubes and hydrogen, and in particular toa supported metallic mixed oxide catalyst system with improvedselectivity for a multi-walled carbon nanotube production process.

STATE OF THE ART

Since the discovery of carbon nanotubes in the beginning of the 90's,intensive research has been conducted for use in different industrialapplications. In fact, carbon nanostructures have shown exceptionalmechanical, electrical, magnetic, optical and thermal properties thatmake them usable in many fields such as artificial muscle, biosensors,composite materials, conductive plastics, flat-panel display,microelectronic devices, extra strong fibres, electron field emission,gas storage, technical textile, protection against flame and antistatic,etc.

Various methods of synthesis have been developed for the production ofcarbon nanotubes with controlled properties including laser ablation,electrical arc discharge and catalytic carbon vapour deposition (CCVD)of hydrocarbons over metallic catalysts.

The CCVD method provides, with respect to other methods, the higheryields and quality of carbon nanotubes and simplifies the manufacturingprocess on an industrial scale. Most of the research carried out in theCCVD technology are presently focussed on developing new catalysts forcontrolling the type (single, double or multi-walled), diameter, lengthand purity of carbon nanotubes. The structural, physical and chemicalproperties of carbon nanotubes have been related to its electricalconducting capacity, mechanical strength and thermal, optical andmagnetic properties.

Document WO-03/004410 discloses a large variety of metal oxides systems(such as Co, Fe, Ni, V, Mo and Cu) and catalyst supports (such asAl(OH)₃, Ca(OH)₂, Mg(OH)₂, Ti(OH)₄, Ce(OH)₄ and La(OH)₃), for thesingle-walled and multi-walled carbon nanotube production. The differentmetals and mixtures of metals in this document were tested on theirselectivity properties, i.e. the ability of the catalyst to selectivelyproduce single, double or multi-walled with respect to a certainproportion of amorphous carbon or fibres formed simultaneously duringthe reaction.

The selectivity properties of the different catalytic systems have beenstudied in the 400° C.-1100° C. temperature range, a hydrocarbon (C₂H₂,C₂H₄ or CH₄)/inert gas (N₂) flow ratio of about 0.1, hydrocarbon spacetime (W/F) of about 12.4 g.h/mol and a reaction time of 60 minutes. Thereported carbon yields in this document varied between 200 wt % and 500wt %, which means that one gram of catalyst produces between 2 and 5grams of carbon.

However, carbon nanotube production on an industrial scale needs furtheroptimisation in selectivity and productivity under relative moderatedreaction temperatures and, in particular, a higher selectivity is neededto produce the desired carbon nanotubes without the formation of othertypes of carbon species (carbon fibres, amorphous carbon, etc).Furthermore, a higher carbon yield not only allows to optimise thecarbon nanotube production per hour and per amount of catalyst butavoids many times its subsequent purification steps which have adetrimental impact on production costs.

SUMMARY OF THE INVENTION

The present invention discloses a catalyst system for the selectiveconversion of hydrocarbons into multi-walled carbon nanotubes andhydrogen comprising a compound of the formula:

(Ni,Co)Fe_(y)O_(z)(Al₂O₃)_(w)

wherein “y” represents the molar fraction of Fe relative to Co and Niandwherein

-   -   0.11≦y≦9.0,    -   1.12≦z≦14.5, and    -   1.5≦w≦64.

Particular embodiments of the present invention disclose at least one ofthe following features:

-   -   preferably, the compound is CoFe_(y)O_(z)(Al₂O₃)_(w) wherein        -   1.5≦y≦2.33,        -   3.33≦z≦4.5, and        -   3≦w≦32;    -   more preferably, the compound is CoFe₂O₄(Al₂O₃)_(w) and        -   4.5≦w≦32;    -   most preferably, the compound is selected from the group        consisting of CoFe₂O₄(Al₂O₃)_(4,5), CoFe₂O₄(Al₂O₃)₁₆ and        CoFe₂O₄(Al₂O₃)₃₂;    -   in a privileged embodiment, the compound is CoFe₂O₄(Al₂O₃)₃₂;    -   the compound is obtained by a thermal treatment of a hydroxide        precursor of the formula (I)

(Ni,Co)Fe_(y)(OH)_(p)(Al(OH)₃)_(q)

wherein

-   -   1.5≦y≦2.33,    -   6.5≦p≦9.0, and    -   3≦q≦128;    -   preferably, said hydroxide precursor is a hydroxide precursor of        the formula:

CoFe₂(OH)_(p)(Al(OH)₃)_(q)

wherein

-   -   7.0≦p≦8.5 and    -   6≦q≦96;    -   more preferably, the hydroxide precursor is a hydroxide        precursor of the formula:

CoFe₂(OH)₈(Al(OH)₃)_(q)

wherein

-   -   9≦q≦64;    -   most preferably, the hydroxide precursor is a hydroxide        precursor of the formula:

CoFe₂(OH)₈(Al(OH)₃)₃₂;

-   -   in a privileged embodiment the hydroxide precursor is a        hydroxide precursor of the formula:

CoFe₂(OH)₈(Al(OH)₃)₆₄.

Additionally, the present invention discloses a process for synthesisingthe hydroxide precursor wherein a reaction between cobalt/nickel, ironand aluminium compounds is carried out according to a process selectedfrom the group consisting of impregnation, co-precipitation, sol-gel andcitrate complexation.

Additional embodiments of the synthesis of the precursor of the presentinvention comprises at least one of the following features:

-   -   impregnation of an aluminium hydroxide with metallic solutions        containing soluble salts of Co, Ni and Fe;    -   the impregnation is a simultaneous impregnation;    -   said aluminium hydroxide is selected from the group consisting        of bayerite and gibbsite;    -   preferably, the aluminium hydroxide is gibbsite;    -   preferably, the gibbsite has a specific surface between 8 and 20        m²/g;    -   aluminium hydroxide is obtainable by a calcination of aluminium        hydroxide at T≧350° C. for 0.5 to 4 hours;    -   the metallic solution containing soluble salts of Co, Ni and Fe        comprises cobalt acetate or cobalt nitrate, nickel acetate or        nickel nitrate, iron acetate or iron nitrate;    -   the cobalt or nickel acetate is selected for (Co/Ni)/(Co/Ni)+Fe        ratios between 0.30-0.40 and cobalt nitrate is selected for the        Co/Co+Fe ratios between 0.30-0.75;    -   the process for synthesising the hydroxide precursor catalyst        comprises the step of co-precipitation of an aluminium hydroxide        with metallic solutions containing soluble salts of Co, Ni and        Fe;    -   the aluminium hydroxide for the co-precipitation step is        selected from the group consisting of bayerite and gibbsite;    -   preferably, the aluminium hydroxide is gibbsite;    -   the process for synthesising the hydroxide precursor catalyst        comprises the additional step of drying the impregnated or        co-precipitated mixed hydroxide at temperatures between 60°        C.-120° C. for 1 to 4 hours;    -   the process for synthesising the hydroxide precursor catalyst        additionally comprises the step of calcinating the impregnated        or co-precipitated mixed hydroxide at temperatures between        350° C. and 800° C. for 10 minutes to 1 hour;    -   the calcination comprises two steps, a first step comprising        heating in a flow of nitrogen at a temperature ranging from        120° C. to 350° C. at a rate of heat between 5° C. to 20° C. per        minute and remaining isothermally at the same conditions between        0.5 to 4 hours, and a second step comprising a heating in a flow        of nitrogen between 450° C. to 700° C. at a rate of heat between        5° C. to 20° C. per minute, and remaining isothermally between        0.5 to 2 hours;    -   the first step comprises heating in a flow of nitrogen at a        temperature ranging from 120° C. to 350° C. at a rate of heat        between 5° C. to 20° C. per minute and remaining isothermally at        the same conditions between 1 to 2 hours and the second step        comprising a heating in a flow of nitrogen between 500° C. to        600° C. at a rate of heat between 5° C. to 20° C. per minute,        and remaining isothermally between 0.5 to 2 hours;    -   the catalyst support particle sizes as determined by XRD        technique, when using the gibbsite variety of aluminium        hydroxide, is between 30-70 nm;    -   the catalyst support grain sizes, when using the bayerite        variety of aluminium hydroxide, is between 20 μm to 70 μm.

Furthermore the present invention discloses a process for the selectiveconversion of hydrocarbons into multi-walled carbon nanotubes andhydrogen comprising the steps of:

-   -   providing a catalyst precursor according to any of claims 5 to        8;    -   activating the catalyst precursor by drying and/or calcination        according to any of the claims 23 or 26;    -   contacting the activated catalyst with a carbon source under        multi-walled carbon nanotube production conditions defined by        the reaction temperature and the reaction space time (W/F);    -   extracting multi-walled nanotubes.

Additional embodiments of the synthesis of the precursor of the presentinvention comprise at least one of the following features:

-   -   the carbon source is an olefin, an alkane or a mixture of them;    -   the olefin is ethylene and/or propylene;    -   the alkane is methane and/or ethane;    -   the alkane mixture is natural gas;    -   the reaction temperature is between 600° C. and 1100° C.;    -   the reaction temperature is between 650° C. and 950° C.;    -   the reaction space time (W/F) of the hydrocarbon is between 0.20        g.h/mole and 0.75 g.h/mole;    -   the reaction space time (W/F) of the ethylene is comprised        between 0.30 g.h/mole and 0.45 g.h/mole;    -   the reaction space time (W/F) of the methane is between 0.20        g.h/mole and 0.45 g.h/mole;    -   the selective conversion of hydrocarbons into multi-walled        carbon nanotubes and hydrogen is performed according to a thin        film catalysed bed process, a moving bed process or a rotary        kiln process.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 represents the transmission electron microscopy (TEM) resultscorresponding to the carbon nanotubes synthesised by using different Coand Fe oxide model compounds.

FIG. 2 represents the XRD patterns corresponding to the(Ni,Co)Fe_(y)O_(z)(Al₂O₃)_(w) catalyst series prepared from differentCo/Co+Fe ratio composition and constant Al₂O₃ composition (w=32).

FIGS. 3 and 4 represent the XRD patterns corresponding to theCoFe₂O₄(Al₂O₃)_(w) catalyst series prepared from different Al₂O₃compositions and constant metal composition (Co/Co+Fe=0.33).

FIG. 5 represents the particle size distribution corresponding todifferent CoFe₂O₄(Al₂O₃)_(w) formulations.

FIG. 6 represents the transmission electron microscopy images of themulti-walled carbon nanotube synthesised at 20 minutes and 60 minutes onCoFe₂O₄(Al₂O₃)₃₂ catalyst.

FIG. 7 represents the X-ray photoelectron spectroscopy (XPS) resultscorresponding to the Co2P energy level of CO₃O₄ model compound.

FIG. 8 represents the X-ray photoelectron spectroscopy (XPS) resultscorresponding to the Co2P energy level of CoFe₂O₄(Al₂O₃)_(4,5),CoFe₂O₄(Al₂O₃)_(7,5), CoFe₂O₄(Al₂O₃)_(10,5) catalysts and the CoFe₂O₄model compound.

FIG. 9 represents the X-ray photoelectron spectroscopy (XPS) resultscorresponding to the Fe2P energy level of CoFe₂O₄(Al₂O₃)_(4,5),CoFe₂O₄(Al₂₃)_(7,5), CoFe₂O₄(Al₂₃)_(10,5) catalysts.

FIG. 10 represents the X-ray photoelectron spectroscopy (XPS) resultscorresponding to the Fe2P energy level of Fe₂O₃, Fe₃O₄ the CoFe₂O₄ modelcompounds.

FIG. 11 represents the effect of the reaction temperature on the carbonyield.

FIG. 12 represents the effect of the reaction temperature on the carbonnanotube outer diameter.

DETAILED DESCRIPTION OF THE INVENTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood to those skilled in the artin the field of catalysis.

The catalyst system disclosed in the present invention is based on amixed oxide catalyst system containing iron, cobalt and/or nickelsupported on aluminium oxide. This catalytic system produces in thepresence of a carbon source under adequate operating conditionsmulti-walled nanotubes. The new mixed oxide catalyst system is providedalong with his preparation process.

Non-restrictive examples of carbon sources are unsaturated or saturatedhydrocarbons, such as acetylene, ethylene, methane or natural gas aswell as cyclic hydrocarbons such as cyclohexane, cyclohexene, aromaticcompounds, linear or branched hydrocarbons.

Compared to the multi-walled carbon nanotube yields with catalysts ofthe prior art and in particular of WO 03/004410, the process of thepresent invention provides for about a 31-times decreased space time,with higher hydrocarbon yields (3.7-times higher) and purity (of about94% vs. 83%). This comparison is given in Table 1a. The hydrocarbonspace-time (W/F) is defined as the weight of the catalyst in gramsdivided by the flow of reactant stream in mole/h at standard temperatureand pressure conditions.

TABLE 1a Comparison between the effectiveness of the prior art catalystsversus the present invention Space time Reaction Temp. Carbon CarbonCatalyst (g · h/mole) time (min) (° C.) yield (%) purity (%) Prior art12.4 60 700 510 83.6 WO-03/ 004410 Present 0.40 20 700 1247 92.6invention (20 min) Present 0.40 60 700 1894 95.0 invention (60 min)

Although the specific procedures and methods as described herein aremainly exemplified for the multi-walled carbon nanotube production fromhydrocarbons, they are merely illustrative for the practice of theinvention.

The process according to the present invention may be carried out as avapour phase reaction. It is well understood that during the course ofthe process of the present invention, diluent inert gasses may be usedsuch as He, N₂ and Ar and equally reducing or oxidising agents such asH₂ or CO₂ may be also added to the gas reaction.

The feedstock may be a single olefin or alkane, a mixture of alkanes, ormixture of olefins, or a mixture of alkane and olefins.

The hydrocarbon and a diluent gas concentration by volume of the feedsupplied to the reactor in the present invention is within a range of50-100 vol % in hydrocarbon and 0-50 vol % in diluent gas, preferablyfrom 60 vol % to 90 vol % in hydrocarbon and from 10 vol % to 40 vol %in diluents.

As previously stated, the conversion process of hydrocarbons to carbonnanotube according to the present invention is carried out as a vapourphase reaction. Accordingly, any apparatus of the type suitable forcarrying out CCVD reactions may be employed for the practice of theprocess. The process may be operated continuously or intermittently andmay employ a thin film catalyst bed, moving bed or the so-calledfluidised catalytic bed with finely divided particles. Table 1b showsthe activity behaviour of the catalyst of the present invention obtainedby different types of catalytic reactors. All tests show carbon yieldand purity higher than 1000% and 90%, respectively. The better resultswere obtained using a moving bed catalytic reactor.

TABLE 1b Activity results of the catalyst of the present inventionobtained using different catalytic reactors Temperature/ Type ofreaction time/ Carbon yield/ reactor Process % C₂H₄ in the feedstockpurity (%) Fix bed Discontinuous 700° C., 20 min, 100% 1247/92.6 Movingbed Continuous 700° C., 20 min, 80% 1550/93.4 Thin film* Discontinuous700° C., 20 min, 100% 1040/90.5 Thin film** Discontinuous 700° C., 20min., 100% 2915/96.7 *Co/Co+Fe = 0.5 **Co/Co+Fe = 0.33

The conversion process of hydrocarbons to carbon nanotube of the presentinvention is carried out at temperature in a range from 500° C. to 1100°C. and preferably in a range of from 650° C. to 950° C.

Pressures others than atmospheric may be employed in the process of thepresent invention; however, the process is usually conducted at or nearatmospheric pressure, since the reaction proceeds well at such pressure.

The W/F values employed in the process of this invention may be selectedfrom a broad operable range that may vary from 0.20 g.h/mole to about0.80 g.h/mole. In the case of converting ethylene into carbon nanotube,a suitable space-time will be within a range from 0.30 g.h/mole to about0.40 g.h/mole (Table 2). The optimum space-time will of course dependupon the hydrocarbon being reacted, the catalyst composition and thereaction temperature, but in general ranges between 0.20 g.h/mole and0.45 g.h/mole.

TABLE 2 Carbon yield as a function of the ethylene space time Space time(g · h/mole) 0.30 0.40 0.55 0.75 Mole of carbon per gram of catalyst1.025 1.039 0.923 0.745

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a catalyst with a metallic systemcomprising a metal selection on an available pool of appropriate metalsincluding mixed metals. As such, by playing on a choice of appropriatemetals, the nature, number and strength of the catalytic sites may bemodulated. Therefore, one may modulate the catalyst's selectivity andthe conversion rate of the starting product according to one's desiredend product.

Determination of the Stoechiometric Structure of the Catalyst

In order to determine the stoechiometric structure of thenickel/cobalt-iron-aluminium oxide catalysts, chemical analysis andthermo-gravimetric measurements have been conducted on the differentprepared samples (Table 3) on the basis of cobalt-iron-aluminium oxidecatalysts.

TABLE 3 Chemical composition and thermo-gravimetric measurementscorresponding to different cobalt- iron-aluminium oxide catalysts withtwo equivalent ways of representation Catalyst % residue (thermo- %residue composition gravimetric analysis) (theoretical) CoFe₂O₄(Al₂O₃)₃₂13,960 13,953 (CoFe_(2.1)Al_(68.6)O_(106.9)) CoFe₂O₄(Al₂O₃)_(10.5)17,665 17,695 (CoFe_(2.0)Al_(21.1)O_(32.7)) CoFe₂O₄(Al₂O₃)_(7.5) 17,33217,358 (CoFe₂Al_(15.0)O_(23.4)) CoFe₂O₄(Al₂O₃)_(4.5) 16,712 16,754(CoFe₂Al_(9.0)O_(14.2)) CoFe₂O₄(Al₂O₃)_(3.0) 16,177 16,231(CoFe₂Al_(6.0)O_(9.5)) CoFe₂O₄(Al₂O₃)_(1.5) 15,308 15,384CoFe₂Al₃O_(4.9)

For all samples, the cobalt, iron and aluminium content was determinedby induced current plasma technique. The results in Table 3 arerepresented as a function of the Fe/Co and Al/Co atomic ratio. Theoxygen content was obtained using the following equation:

Owt %=100%−Alwt %−Fewt %−Cowt %.

The O/Co atomic ratio is also represented for the different catalystcompositions.

The loss of weight after calcination of the samples at 700° C. wasdetermined by thermo-gravimetric measurements. The theoretical residuewas estimated taking into account the initial weight of Al(OH)₃, Fe(NO₃)₃ and Co(AOC)₂, used for the preparation of catalyst and assumingthat these compounds are transformed into CoFe₂O₄ and Al₂O₃ aftercalcination at 700° C.

The relation between aluminium and oxygen atoms can be obtained byplotting O/Co vs Al/Co atomic ratio for the different samples. Thestraight line is obtained which correspond to the following relation:

O atom grams=1.5Al atom grams+4.0

The general equation obtained experimentally is the following:

CoFe₂O₄(Al₂O₃)_(w)

The experimental and theoretical residue values obtained are practicallythe same, which strongly suggest that a CoFe₂O₄-like phase supported onalumina is formed after calcination of the Co—Fe—Al precursor hydroxidecatalysts.

In the present invention the conversion process of hydrocarbons tocarbon nanotube involves a mixed oxide catalyst comprising aluminium andcombinations of transition metals. The precursor of said mixed oxidecatalyst comprises a hydroxide precursor of the formula (I)

Ni,Co)Fe_(y)(OH)_(p)(Al(OH)₃)_(q)  (1)

wherein

-   -   1.5≦y≦2.33,    -   6.5≦p≦9.0, and    -   6≦q≦128.

Advantageously, the catalyst precursor responds to the following generalformulation (2)

CoFe₂(OH)₈(Al(OH)₃)₆₄  (2)

wherein

-   -   q=64

A precursor hydroxide catalyst preparation process by mineral wayinvolves a reaction between a salt of metal and an aluminium hydroxide.In the case of the present invention, the catalyst preparation processinvolves the reaction between cobalt or nickel compounds and an ironcompound with an aluminium compound, followed by a drying step and acalcination step in order to obtain a mixed hydroxide compound thatcorrespond to the above general formulation (2).

A reaction between cobalt/nickel, iron and aluminium compounds may becarried out by mineral way, comprising impregnation, co-precipitation,sol-gel and citrate complexation methods.

A reaction between cobalt, iron and aluminium compounds is suitablyachieved by impregnation or co-precipitation, which may be carried outby contacting a cobalt/nickel salt, for instance cobalt/nickel acetate(Co/Ni)(AOC)₂ or cobalt nitrate (Co/Ni)(NO₃)₂, iron acetate Fe(AOC)₃ oriron nitrate Fe(NO₃)₃, with an aluminium hydroxide, for instance Al(OH)₃or γ-AlOOH.

A suitable particle size distribution is such that the alumina hydroxidesupport particles have a size within a range from 5 microns to 70microns. In this particle size range, the catalytic reaction is notlimited by internal diffusion processes. The effect of grain sizes of abayerite (Al(OH)₃) used for the preparation of a Co—Fe supportedcatalyst on the activity properties is showed in Table 4.

TABLE 4 Effect of grain sizes of the alumina hydroxide support on themulti-walled carbon nanotube production Particle size (μ) <20 20-70 >70Mole of C/g. catalyst 1.025 0.998 0.751

A reaction between Co/Ni, Fe and Al may also be carried out by organicsol-gel way. In this case, the reaction may involve an aluminiumalkoxide, for instance aluminium tributoxyde; and a cobalt/nickelalkoxide as well as an iron alkoxide.

Using the complexation catalyst preparation method, the suitable Co/Ni,Fe and Al compounds may be metallic acetyl-acetonate salts[C₁₀H₁₉CO⁺²O₄, C₁₅H₂₁Fe⁺³O₆ and C₁₅H₂₁Al⁺³O₆] in an acid organicreaction environment such as citric acid.

Following a drying operation, the hydroxide is then calcinated to forman oxide precursor catalyst. Said drying operation may be carried out attemperatures from 30° C. to 150° C. Particular useful temperatures forsaid drying operation range from 60° C. to 120° C. using for example aconventional dryer, a ring dryer or a spray dryer equipment.

Calcination may be achieved in two steps. A first step typicallycomprises heating at a temperature ranging from 120° C. to 350° C. at arate of heat between 5° C. to 20° C. per minute in a flow of nitrogen,remaining isothermally at the same conditions between 0.5 to 4 hours,preferably between 1 to 2 hours. A second step may comprise a heatingbetween 450° C. to 700° C. at a rate of heat comprised between 5° C. to20° C. per minute in a flow of nitrogen, preferably between 500° C. and600° C., remaining isothermally between 0.5 to 2 hours. Calcination maybe achieved in a conventional oven, rotary kiln or any of the typesuitable for carrying out the calcination pre-treatment.

The calcination of the precursor hydroxide catalyst at temperaturesbetween 300° C. and 700° C. produce structural modifications of Co, Feand Al hydroxide phases. The aluminium hydroxide is decomposed inalumina (γ-Al₂O₃) and H₂O, while Co and Fe ions are transformed inindifferent oxidised phases such as α-Fe₂O₃, Fe₃O₄, CoFe₂O₄, CO₃O₄,CoAl₂O₄, FeAl₂O₄. The nature and composition of these Co and Fe phasestrongly depends on the Co/Fe atomic ratio composition, the nature ofthe catalyst support and the calcination temperature.

The loss of weight of the catalyst due to H₂O molecules removal duringcalcination varies between 30 wt % and 40 wt %, and this range mainlydepends on the type of alumina hydroxide used and the metal loading. Thecalcinated precursor oxide catalyst described in the present inventionresponds to the following general simplified formulae (3)

(Ni,Co)Fe_(y)O_(z)(Al₂O₃)_(w)  (3)

wherein “y” represents the number of Fe mole relative to Co and/or Nimole andwherein

-   -   1.5≦y≦2.33,    -   3.33≦z≦4.5, and    -   4.5≦w≦48.

Control of selectivity is one of the major roles governed byheterogeneous catalysts. Selectivity depends on the nature, surfacedispersion and particle sizes distribution of Co and Fe phases as wellas the textural, physico-chemical and acid-base properties of thecatalyst support. In general, acid supports leads to the formation ofamorphous carbon species (coke, graphitic carbon, etc) by crackingreaction mechanisms during the hydrocarbon decomposition at hightemperature.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The conversion process of hydrocarbons to carbon nanotube according tothe present invention comprises the use of a(Ni,Co)Fe_(y)O_(z)(Al₂O₃)_(w) catalyst which is characterised by a highperformance and selectivity under reaction conditions.

The following non-restrictive examples are illustrative for preferredembodiments of the invention.

Catalysts were tested in the decomposition reaction of ethylene at 700°C., W/F=0.40 g.h/mole of ethylene, and reaction time of 20 minutes.

The carbon yield was determined experimentally from the followingrelationships:

carbon deposit(wt %)=100(m _(tot) −m _(cat))/m _(cat)

where m_(tot) and m_(cat) are the total weight of the product afterreaction and the mass of catalyst before reaction, respectively.

Example 1

A series of CoFe/Al₂O₃ catalysts were prepared by co-impregnation ofAl(OH)₃ support with Co(AOC)₂+Fe(NO₃)₃ solutions. The total metalloading (Co, Fe or a mixed Co+Fe) was 3.2 wt % for all samples.

In a first embodiment of the present invention a(NiCo)Fe_(y)O_(z)(Al₂O₃)_(w) catalyst with a Co/Co+Fe compositionbetween 0.2 and 0.8 including the outer limits, was prepared. A furtherembodiment relates to an optimal Co/Co+Fe ratio composition between 0.30and 0.50 including the outer limits. The most efficient Co/Co+Fecomposition ratio is between 0.30-0.40 (Table 5a).

TABLE 5a Influence of the Co/Co+Fe composition in the catalyst on thecarbon yield Co/Co+Fe 0 0.25 0.33 0.40 0.50 0.75 1.0 Carbon yield (%)191 924 1200 1156 982 610 333 Carbon purity (%) 65.6 90.2 92.3 92.0 90.885.9 76.9 Thermal stability — 514 520 518 516 488 — (° C.)

In a second series of experiments, a (Ni,Co)Fe_(y)O_(z)(Al₂O₃)_(w)catalyst with different Ni/Ni+Fe ratio compositions was prepared. Inthis case, nickel nitrate salt was used in the preparation of thesamples. In Table 5b, one can observe that the optimal carbon yield andpurity is obtained for Ni/Ni+Fe ratio composition between 0.25 and 0.33.

TABLE 5b Influence of the Ni/Ni + Fe composition in the catalyst on thecarbon yield Ni/Ni + Fe 0.25 0.33 0.50 0.75 Carbon yield (%) 975 928 663108 Carbon purity (%) 90.7 90.3 86.9 51.9

Example 2

In this example, a cobalt nitrate salt was used instead of cobaltacetate for the preparation of the catalyst. The results of themulti-walled nanotube synthesis are shown in Table 5c. It is confirmedthat an optimal carbon yield is obtained for a Co/Co+Fe composition inthe catalyst of about 0.33. However, the carbon yield is higher thanthat observed for those catalysts prepared starting from Co(AOC)₂ saltand Co/Co+Fe ratio composition between 0.50 and 0.75.

TABLE 5c Influence of the Co/Co+Fe composition in the catalyst on thecarbon yield Co/Co+Fe 0 0.25 0.33 0.50 0.75 Carbon yield (%) 191 7031137 1081 1073

The catalytic behaviour observed when using different cobalt salts canbe explained as follows: iron hydroxide species (Fe(OH)₃) precipitatesat pH≧2.5. The addition of Co(AOC)₂ to the iron solution increases thepH by H₃O⁺ ions consumption, according to the following equation:

CH₃COO⁻+H₃O⁺<->CH₃COOH+H₂O

The impregnating solution becomes unstable for Co/Co+Fe ratiocomposition≧0.50. A dark brown precipitate was formed, thus the activityof the catalysts was affected by the heterogeneous distribution of themetals. By contrast, by using cobalt nitrate instead of Co(AOC)₂, theimpregnating solutions were clear and stables several days and for allCo/Co+Fe ratio compositions because the pH remained below 2.0.

Example 3

In another series of experiments, the catalysts were prepared withdiffering amounts of aluminium atoms and Co/Co+Fe ratio compositionequal to 0.33. The latter correspond to CoFe₂O₄(Al₂O₃)_(w), simplifiedformulae. The activity results of the catalyst series are shown in Table6.

Two metal compositions in the catalyst deliver optimal catalyticactivity. They are 9.5 wt % and 27.1 wt % of supported metals in thecatalysts and correspond to CoFe₂O₄(Al₂O₃)₁₆ and CoFe₂O₄(Al₂O₃)_(4,5)atomic ratios, respectively. The CoFe₂O₄(Al₂O₃)_(4,5) was obtained byco-precipitation of Co, Fe and Al, thus Al(OH)₃ becomes soluble at verylow solution pH (pH<1.5). In this case, Al(OH)₃ is used asco-precipitating agent. The CoFe₂O₄(Al₂O₃)₁₆ was prepared byimpregnation of the Al(OH)₃ support from a Co(AOC)₂ and Fe(NO₃)₃solution.

TABLE 6 Effect of the Al composition in the catalyst on the carbon yieldAtomic ratio composition % of active phase and compound between in thecatalyst Carbon brackets (Co + Fe/Co + Fe + Al₂O₃) yield (%)CoFe₂O₄(Al₂O₃)_(1.5) 52.7 1382 CoFe₂O₄(Al₂O₃)₃ 35.8 1385CoFe₂O₄(Al₂O₃)_(4.5) 27.1 1432 CoFe₂O₄(Al₂O₃)₆ 21.8 1312CoFe₂O₄(Al₂O₃)_(7.5) 18.2 1238 CoFe₂O₄(Al₂O₃)₉ 15.7 1252CoFe₂O₄(Al₂O₃)_(10.5) 13.8 1191 CoFe₂O₄(Al₂O₃)₁₆ 9.5 1430CoFe₂O₄(Al₂O₃)₃₂ 5.0 1200 CoFe₂O₄(Al₂O₃)₆₄ 2.5 430

Example 4

The effect of the order of addition of Co and Fe elements during thecatalyst preparation was also investigated. Consecutive and simultaneousimpregnation steps were carried out starting from Co(AOC)₂ and/orFe(NO₃)₃ solutions.

Table 7 shows the carbon yield for the samples prepared by consecutiveimpregnation of metals (Co->Fe or Fe->Co) relative to the catalystprepared by co-impregnation (Co+Fe). Consecutive impregnation Fe->Coprovides equal performance than co-impregnation. However, the Co->Feimpregnation sequence produces catalysts with lower carbon nanotubeyields. From the industrial standpoint, the co-impregnation is thedesired technique of preparation of the catalyst of the presentinvention.

TABLE 7 Effect of the order of addition of Co and Fe solutions on therelative carbon yield Impregnation sequence Co -> Fe Fe -> Co Relativecarbon yield 0.754 0.987

Example 5

X-ray diffraction (XRD) and Mössbauer spectroscopy analysis conducted ondifferent mechanical mixture of Co and Fe salts have evidenced thepresence of α-Fe₂O₃, Fe₃O₄, Co₃O₄ and CoFe₂O₄ phases after calcinationat temperatures higher than 400° C. The relative proportion of thesephases depends on the Co/Co+Fe composition. For instance, the Co and Fephases observed at different Co/Co+Fe are summarised in Table 8.

TABLE 8 Co and Fe oxidic phases observed by XRD and Mossbauerspectroscopy techniques as a function of the Co/Co + Fe ratiocomposition 0-0.1 0.1-0.4 0.4-0.8 0.8-1.0 α-Fe₂O₃ X X X X Fe₃O₄ X XCo₃O₄ X X CoFe₂O₄ X X

To understand the role of the Co/Co+Fe ratio on the catalytic propertiesof the catalysts, a number of tests were conducted using theabove-mentioned cobalt and/or iron oxide model compounds. The activityresults are presented in Table 9. It is observed that the CoFe₂O₄ mixedphase provide higher carbon yields than cobalt or iron oxides. Under theexperimental reaction conditions, MW-CNT was only observed bytransmission electron microscopy technique for CoFe₂O₄ and Co₃O₄compounds. Using cobalt or iron oxides, amorphous carbon (such as metalcarbide or oxide metallo carbide) were only observed.

TABLE 9 Co and Fe oxide phases observed by XRD and Mossbauerspectroscopy techniques as a function of the Co/Co + Fe ratiocomposition Carbon yield (%) α-Fe₂O₃ 93 Fe₃O₄ 132 Co₃O₄ 137 CoFe₂O₄ 474

In order to verify the presence of the above Co and Fe oxide modelcompounds, in FIGS. 2, 3 and 4 is shown XRD diffraction patternscorresponding to the different CoFe₂O_(z)(Al₂O₃)_(w) catalystformulations. A CoFe₂O₄-like phase was identified for the Co/Co+Fe ratiocomposition between 0.30-0.75 (FIG. 2) and the peak intensities ishigher for the Co/Co+Fe ratio composition=0.33 (FIG. 3). For the samplesprepared with different aluminium content and Co/Co+Fe=0.33, the signalscorresponding to the CoFeO₄-like phase are slightly shifted towardhigher 2e values. This suggests that an CoFe₂Al_(t)O_(n) cluster isformed by a solid stated reaction during calcination at highertemperature.

To investigate the role of aluminium atoms in the CoFe₂O_(z)(Al₂O₃)_(w)catalyst, we conducted experiments from mechanical mixer ofCo(AOC)₂+Fe(NO₃)₃ and Al(OH)₃ in a ball-mill for 30 minutes followed bya drying (120° C., 30 minutes) and calcination (700° C., 15 minutes)steps. The activity of the different prepared samples is shown in Table10. It is concluded from the obtained results that both the Co/Co+Feratio composition and the presence of Al(OH)₃ play an important role onthe performance of the carbon nanotube production catalyst. The resultsof Table 10 also indicate that the co-impregnation is the best methodfor preparing the CoFe₂Al_(t)O_(n) carbon nanotube production catalyst.

TABLE 10 Activity of the catalysts prepared by different methodsCatalyst preparation Catalyst preparation Carbon procedure methodCo/Co + Fe yield (%) Co(AOC)₂ + Fe(NO₃)₃ Mechanical mixture 0.33 311Co(AOC)₂ + Fe(NO₃)₃ + Mechanical mixture 0.50 894 Al(OH)₃ Co(AOC)₂ +Fe(NO₃)₃ + Mechanical mixture 0.33 932 Al(OH)₃ Co(AOC)₂ + Fe(NO₃)₃ +Impregnation 0.33 1200 Al(OH)₃

Example 6

The effect of the type of aluminium hydroxide support on the catalyticproperties of the CoFe₂O₄(Al₂O₃)₃₂ catalyst is shown in Table 11. Thesesaluminium compounds form different crystallographic structures and theyshow differences in surface area and acid-base properties.

TABLE 11 Effect of the nature of catalyst support on the carbon yieldgibbsite gibbsite bayerite alumina Type of support gibbsite 1 2 3 (20-60μm) γ-Al₂O₃ pH in solution 7.8 7.9 8.1 9.0 7.5 (after 24 h) Surface aream²/g 186 193 238 198 250 Carbon yield (%) 140 177 1200 1130 283 VP(cc/g) 0.25 0.26 0.28 0.25 0.71 Average diameter 39 39 37 38 74 of pore(A°) particle size >150 >150 48 120 — (nm) as determined by XRD

The results of Table 11 clearly show that Al(OH)₃ provides moreeffective catalysts than AlOOH and γ-Al₂O₃ supports in the carbonnanotube production. The basic character of the Al(OH)₃ support enhancesthe carbon yield. Gibbsite and bayerite are suitable Al(OH)₃ supports orprecipitating agents for the preparation of the CoFe2Oz(Al2O3)wcatalysts of the present invention. Smaller particle sizes of theAl(OH)₃ support provide higher BET surface area, therefore Co and Fesurface metal dispersion and CNT's yield.

Example 7

Carbon nanotube diameter is influenced by different parameters such asthe reaction temperature, the reaction time and the metallic particlesize. Table 12 shows the variation of MWNT carbon nanotube diameter, asdetermined by transmission electron microscopy technique, as a functionof the time of reaction for two catalyst formulations. The resultsclearly show that the carbon nanotube diameter progressively increasesas a function of the reaction time.

FIG. 5 shows the variation of catalyst particle sizes distribution,determined by light scattering technique, for differentCoFe₂O₄(Al₂O₃)_(w) compositions. As expected, the particle sizesincrease when the Co and Fe composition in the catalyst increases,therefore higher carbon nanotube diameters are obtained.

TABLE 12 Variation of the multi-walled carbon nanotube diameter as afunction of the reaction time and the catalyst CoFe₂O₄(Al₂O₃)_(w)composition Reaction time (min.) 10 20 30 60 CoFe₂O₄(Al₂O₃)_(4.5) 13.0nm 13.5 nm 14.7 nm 14.9 nm CoFe₂O₄(Al₂O₃)₃₂  8.5 nm 10.6 nm 10.8 nm 12.3nm

Transmission electron microscopy images of multi-walled carbon nanotubesynthesised on the CoFe₂O₄(Al₂O₃)_(w) catalyst after 20 minutes and 60minutes of reaction are shown in FIG. 6.

Example 8

In order to characterise the Co and Fe phases present in theCoFe₂O_(z)(Al₂O₃)_(w) catalysts, X-ray photoelectron spectroscopyanalysis were carried out on CoFe₂O₄(Al₂O₃)_(4,5), CoFe₂O₄(Al₂O₃)_(7,5),CoFe₂O₄(Al₂O₃)_(10,5) and on CO₃O₄, Fe₂O₃ and CoFe₂O₄ model compounds.These results are shown is FIGS. 7-10, respectively. From these figures,it is clear that the peak position and the shape of the XPS signals aresimilar for the three CoFe₂O_(z)(Al₂O₃)_(w) catalyst compositions. Theyalso correspond to the CoFe₂O₄ model compound, which is in agreementwith the XRD results showed in FIG. 2-4.

Example 9

We investigated the effect of the reaction temperature (600-800° C.temperature range) on the carbon nanotube yield and outer diameter usingthe CoFe₂O₄(Al₂O₃)₃₂ catalyst formulation. The results of theseexperiments are shown in FIGS. 11 and 12, respectively. Optimal carbonyield is obtained at 700° C. (1400%). Under this reaction condition, theCNT diameter is about 9 nm. Very thin multi-wall CNT having outerdiameter between 6-7 nm are produced at reactions temperatures between650-675° C. At T>700° C., the CNT diameter increases while the CNT yielddecreases continuously due to the sintering of both the active phase andcatalyst support.

1. A catalyst system for the selective conversion of hydrocarbons intomulti-walled carbon nanotubes and hydrogen comprising a compound of theformula:(Ni,Co)Fe_(y)O_(z)(Al₂O₃)_(w) wherein “y” represents the molar fractionof Fe relative to Co and Ni and wherein 0.11≦y≦9.0, 1.12≦z≦14.5, and1.5≦w≦64.
 2. The catalyst system of claim 1, wherein the compound isCoFe_(y)O_(z)(Al₂O₃)_(w) and wherein 1.5≦y≦2.33, 3.33≦z≦4.5, and 3≦w≦32.3. The catalyst system of claim 1, wherein the compound isCoFe₂O₄(Al₂O₃)_(w) and 4.5≦w≦32
 4. The catalyst system of claim 1,wherein the compound is selected from the group consisting ofCoFe₂O₄(Al₂O₃)_(4,5), CoFe₂O₄(Al₂O₃)₁₆ and CoFe₂O₄(Al₂O₃)₃₂.
 5. Thecatalyst of claim 1, wherein the compound is CoFe₂O₄(Al₂O₃)₃₂.
 6. Thecatalyst system of any of claims 1 to 5, wherein the compound isobtained by a thermal treatment of a hydroxide precursor of the formula(I)(Ni,Co)Fe_(y)(OH)_(p)(Al(OH)₃)_(q) wherein 1.5≦y≦2.33, 6.5≦p≦9.0, and3≦q≦128.
 7. The catalyst system of claim 6, wherein said hydroxideprecursor is a hydroxide precursor of the formula:CoFe₂(OH)_(p)(Al(OH)₃)_(q) wherein 7.0≦p≦8.5 and 6≦q≦96.
 8. The catalystsystem of claim 7, wherein said hydroxide precursor is a hydroxideprecursor of the formula:CoFe₂(OH)₈(Al(OH)₃)_(q) wherein 9≦q≦64.
 9. The catalyst system of claim8, wherein said hydroxide precursor is a hydroxide precursor of theformula:CoFe₂(OH)₈(Al(OH)₃)₃₂
 10. The catalyst system of claim 8, wherein saidhydroxide precursor is a hydroxide precursor of the formula:CoFe₂(OH)₈(Al(OH)₃)₆₄
 11. A process for synthesising the hydroxideprecursor of claim 6, wherein a reaction between cobalt/nickel, iron andaluminium compounds is carried out according to a process selected fromthe group consisting of impregnation, co-precipitation, sol-gel andcitrate complexation.
 12. The process of claim 11 comprising the step ofimpregnation of an aluminium hydroxide with metallic solutionscontaining soluble salts of Co, Ni and Fe.
 13. The process of claim 12,wherein said impregnation is a simultaneous impregnation.
 14. Theprocess of claim 12, wherein said aluminium hydroxide is selected fromthe group consisting of bayerite and gibbsite.
 15. The process of claim12, wherein said aluminium hydroxide is gibbsite.
 16. The process ofclaim 12, wherein said aluminium hydroxide is gibbsite with a specificsurface between 8 and 20 m²/g.
 17. The process of claim 14, wherein saidaluminium hydroxide is obtainable by a calcination of aluminiumhydroxide at T≧350° C. for 0.5 to 4 hours
 18. The process of claim 11,wherein said metallic solution comprises cobalt acetate or cobaltnitrate, nickel acetate or nickel nitrate, iron acetate or iron nitrate.19. The process of claim 11, wherein cobalt or nickel acetate isselected for (Co/Ni)/(Co/Ni)+Fe ratios between 0.30-0.40 and cobaltnitrate is selected for the Co/Co+Fe ratios between 0.30-0.75.
 20. Theprocess for synthesising the hydroxide precursor catalyst of claim 6comprising the step of co-precipitation of an aluminium hydroxide withmetallic solutions containing soluble salts of Co, Ni and Fe.
 21. Theprocess of claim 20, wherein said aluminium hydroxide is selected fromthe group consisting of Bayerite and Gibbsite.
 22. The process of claim20, wherein said aluminium hydroxide is gibbsite.
 23. The process ofclaim 20, comprising the additional step of drying the impregnated orco-precipitated mixed hydroxide at temperatures between 60° C.-120° C.for 1 to 4 hours.
 24. The process of claim 23, additionally comprisingthe step of calcinating the impregnated or co-precipitated mixedhydroxide at temperatures between 350° C. and 800° C. for 10 minutes to1 hour.
 25. The process of claim 24, wherein the calcination comprisestwo steps, a first step comprising heating in a flow of nitrogen at atemperature ranging from 120° C. to 350° C. at a rate of heat between 5°C. to 20° C. per minute and remaining isothermally at the sameconditions between 0.5 to 4 hours, and a second step comprising aheating in a flow of nitrogen between 450° C. to 700° C. at a rate ofheat between 5° C. to 20° C. per minute, and remaining isothermallybetween 0.5 to 2 hours.
 26. The process of claim 24, wherein the firststep comprises heating in a flow of nitrogen at a temperature rangingfrom 120° C. to 350° C. at a rate of heat between 5° C. to 20° C. perminute and remaining isothermally at the same conditions between 1 to 2hours and the second step comprising a heating in a flow of nitrogenbetween 500° C. to 600° C. at a rate of heat between 5° C. to 20° C. perminute, and remaining isothermally between 0.5 to 2 hours.
 27. Theprocess of any of the previous claims, wherein the catalyst supportparticle sizes as determined by XRD technique, when using the gibbsitevariety of aluminium hydroxide, is between 30-70 nm.
 28. The process ofany of the previous claims, wherein the catalyst support grain sizes,when using the bayerite variety of aluminium hydroxide, is between 20 μmto 70 μm.
 29. A process for the selective conversion of hydrocarbonsinto multi-walled carbon nanotubes and hydrogen comprising the steps of:providing a catalyst precursor according to any of claims 5 to 8;activating the catalyst precursor by drying and/or calcination accordingto any of the claims 23 or 26; contacting the activated catalyst with acarbon source under multi-walled carbon nanotube production conditionsdefined by the reaction temperature and the reaction space time (W/F);extracting multi-walled nanotubes.
 30. The process of claim 29, whereinthe carbon source is an olefin, an alkane or a mixture of them.
 31. Theprocess of claim 29, wherein the olefin is ethylene and/or propylene.32. The process of claim 29, wherein the alkane is methane and/orethane.
 33. The process of claim 29, wherein the alkane mixture isnatural gas.
 34. The process of claim 29, wherein the reactiontemperature is between 600° C. and 1100° C.
 35. The process of claim 29,wherein the reaction temperature is between 650° C. and 950° C.
 36. Theprocess of claim 29, wherein the reaction space time (W/F) of thehydrocarbon is between 0.20 g.h/mole and 0.75 g.h/mole.
 37. The processof claim 29, wherein the reaction space time (W/F) of the ethylene iscomprised between 0.30 g.h/mole and 0.45 g.h/mole.
 38. The process ofclaim 32, wherein the reaction space time (W/F) of the methane isbetween 0.20 g.h/mole and 0.45 g.h/mole.
 39. The process of claim 29,wherein the selective conversion of hydrocarbons into multi-walledcarbon nanotubes and hydrogen is performed according to a thin filmcatalysed bed process, a moving bed process or a rotary kiln process.